1. Field of the Invention
The present invention relates to an ultrasound diagnostic apparatus of the B-scan type which forms pictorial images of two-dimensional cross sections through a biological body on a cathode ray tube to yield information about the internal structure of the body.
2. Description of the Prior Art
In recent years much effort has been spent on improving ultrasound techniques used in medical diagnosis. While the one-dimensional A-scan technique has proved to be very useful as a diagnostic tool for several parts of the body, the two-dimensional B-scan method has shown advantages over the A-scan method in several respects. One prior art B-scan method involves the use of a sector scanner discussed by J. C. Sommer in Ultrasonics, July 1962, pages 153-159, in which the beam transmitted from a linear array of electromechanical transducers is deflected through an angle by means of an electrically variable delay circuit which introduces a variable delay time as a function of time. This sector scan permits transmission of ultrasonic energy through the narrow spaces between the bone structures of adjacent ribs into the heart of the body to investigate its vertical cross sections.
Another B-scan method involves the use of a linear scanner as disclosed in U.S. Pat. No. 3,881,466 titled "Ultrasonic Cross-sectional Imaging System", in which the ultrasonic beam transmitted from a selected group of transducers, is shifted linearly through a linear array of such transducers. However, the prior art scanners have proved to be unsatisfactory in terms of resolution and signal-to-noise ratio.
U.S. Pat. No. 4,101,795, granted to Fukumoto et al and assigned to the same assignee as the present invention, discloses a piezoelectric transducer composed of a ceramic planar body with a width-to-thickness ratio equal to or smaller than 0.8 having an electromechanical coupling coefficient close to k.sub.33 or k'.sub.33. This transducer provides a vibration mode which is free from unwanted vibrations within the active frequency bandwidth due to the small width as compared to prior art transducers.